Recently, there has been widely used, a technique for stirring molten metal such as molten steel by use of an electromagnetic force in order to improve the quality of the cast slab or bloom, particularly, the solidification structure thereof in the field of the continuous casting of steel or the like. Although the purpose of such use of electromagnetic stirring is different depending upon the place of installation, when the electromagnetic stirring is applied (for example, to a mold for continuous casting), it lies mainly in flowing molten steel to promote floating of bubbles and inclusions and hence removal of bubble defects such as pinholes and blow holes and non-metallic inclusions. Further, the electromagnetic stirrer arranged at the secondary cooling zone beneath the mold is aimed mainly at the increase of equiaxed crystal zone ratio.
Since the idea per se that the electromagnetic stirrer is attached to the continuous caster is a technique quite recently adopted, a concrete structure of the stirrer has not yet been developed. Therefore, a concrete example of the stirrer arranged together with the guide means beneath the mold which is the subject of the invention, is not clearly found in the prior art, but the arrangements of the stirrer just beneath the mold as shown in FIGS. 1 and 2 which show an electromagnetic stirring apparatus, a mold, and a cast strand supporting just beneath the mold.
Such a guide means arranged just beneath the mold is necessary for preventing the cast strand having a thin solidification shell just beneath the outlet of the mold from being broken or deformed due to the unsolidified molten steel remaining in the interior thereof, and also preventing the friction against the lower end portion of the inner wall of the mold. In general, foot rollers (FIG. 1) are arranged in rows or guide plates (FIG. 2) are adopted. Also, support rolls 5 are arranged in rows (FIGS. 1 and 2) for supporting the cast strand.
FIG. 1 is an embodiment of the guide means designed as foot rollers 3 for supporting the cast strand just beneath the mold 2, behind (on the outer side) which is arranged a ring-shaped coil 4 of a rotating magnetic field system. In this embodiment, the coil 4 is so remote from the cast strand that the magnetic force is largely reduced to drop the stirring force, an induced current is produced in the foot rolls 3 to cause troubles such as electrolytic corrosion and the like, and further it is structurally difficult to conduct spray cooling on a steel strand 1.
FIG. 2 is an embodiment of the guide means in which guide plates 6 for the protection of the coil, which also serve to guide the cast strand, are arranged in front of the coil 4 of the rotating magnetic field system. Although this embodiment has the advantage that it is possible to put the coil 4 close to the cast strand 1, the guide plates 6 can not cool the cast strand 1, so that there is a risk of causing abnormal bulging of the cast strand 1 due to the lack of sufficient cooling. Moreover, if the guide plate 6 is made into a water-cooled structure capable of cooling the cast strand, the thickness thereof would need to be greater, hence losing the advantage achieved by closeness of the coil to the cast strand.
It is, therefore, an object of the invention to provide an electromagnetic stirring apparatus with a compact structure which is free from the problems encountered by the conventional electromagnetic stirrer arranged beneath the mold, and gives effectively a stirring force required for the improvement of the quality together with a guiding function inherent to the water-cooled guide plate.
Further, it is another object of the invention to provide an electromagnetic stirring apparatus with a compact structure which is free from the problems encountered by the conventional electromagnetic stirrer arranged beneath the mold, performs a satisfactory cooling while developing an inherent guiding function, and effectively gives a stirring force required for the improvement of the quality.